Thin-film transistors (TFTs) are widely used as switching elements or driving elements in active matrix display devices such as liquid crystal display devices and organic electroluminescent (EL) display devices.
In recent years, there has been a demand for display devices having larger screen sizes and being manufacturable at lower cost. In general, using thin-film transistors that have a bottom-gate type structure in which a gate electrode is formed on the substrate side of a channel layer and that use in combination an amorphous or microcrystalline channel layer made of Si or InGaZnO makes it possible to easily reduce cost. The bottom gate-type thin-film transistors are roughly divided into two types: channel etched type thin-film transistors in which the channel layer is etched, and channel protection type (etching stopper type) thin-film transistors in which the channel layer is protected from etching.
The channel protection type thin-film transistors are capable of preventing damage to the channel layer due to an etching process and suppressing variations in characteristics and changes over time in characteristics in the substrate surface. The channel protection type thin-film transistors are more advantageous in terms of speedup and high definition because reducing the thickness of a gate insulating film and increasing the thickness of a channel protective layer can reduce parasitic resistance components generated by crossing of interconnect lines and can reduce a time constant related to data writing while improving the on-state characteristics of the TFTs.
For example, Patent Literature (PTL) 1 discloses a channel protection type thin-film transistor that uses a microcrystalline semiconductor thin film as a channel layer. The thin-film transistor of PTL 1 has a source electrode and a drain electrode on a channel protective layer. The source electrode and the drain electrode are electrically connected to a semiconductor layer through contact holes formed in the channel protective layer.